NATO’s member states are willing to defend one another, and they have the troops and the equipment to do so. But quickly getting those troops and equipment to their destination is a different matter altogether. In some new NATO member states, bridges and railroads are simply not suitable for large troop movements. But one thing frustrates commanders even more: the arduous process of getting permission to move troops across borders.

“I was probably naïve,” admits Lieutenant General Ben Hodges, the commander of the U.S. Army in Europe. “I assumed that because these were NATO and EU countries we’d just be able to move troops. But ministries of defense are not responsible for borders.”

At their upcoming summit in Warsaw, NATO members will discuss joint responses to Russian aggression, and they are likely to agree to station four battalions—totaling about 4,000 troops—in the Baltic states and Poland. But with Russia forming two new divisions in its western military region, which borders the Baltic states, 4,000 forward-stationed troops may not be enough to deter a potential attack. (A division consists of 10,000 to 20,000 troops.)

Ints Kalnins / Reuters

U.S. troops drive during tactical road march Dragoon Ride II near Daugavpils, Latvia, June 6, 2016.

Indeed, “if NATO’s emerging forward enhanced presence is going to be more than a symbolic tripwire,” Ian Brzezinski, a deputy assistant secretary of defense under George W. Bush who is now a senior fellow at the Atlantic Council, told me, “NATO will have to conduct brigade- to division-level enforcement exercises in the Baltic region.”

And beyond deterrence, if a member state were attacked, NATO commanders would need to quickly pull together troops from different corners and bring them across Europe to their destination, which would most likely be a new NATO member state.

And there’s the complication. Moving troops across Europe requires permission at each border. “During the Cold War, we had pretty good plans to rapidly move across borders, but until [the 2014 NATO summit in] Wales we didn’t have similar plans for new NATO member states,” says a NATO official knowledgeable with